Posting periods are defined in the Fiscal year variant and the SAP system usually proposes the current date as posting date. To prevent the documents getting posted on to an incorrect posting period, you can close certain posting periods.
You open the posting period by entering a range in the posting period variant that encompasses this period and you can have as many periods open as desired. As many periods as required can be opened simultaneously. However, only two periods can be open at the same time during closing. Posting periods can also be handled differently for different account types. At line level the system checks the account type of the posting key to ensure that the period is open for assigned account types.
Posting period variant that contains the open periods must be maintained manually. SAP system uses on posting transaction for several different postings, e.g: G/L acc posting, Customer/Vendor invoice posting, Vendor/Customer credit memo postings. Please note, if you do not define a doc type the system proposed the standard for example KR for Vendor Invoices.